1. Technical Field
This invention relates generally to digital-to-analog (D/A) converters and more particularly to a three-level D/A converter for low consumption of power.
2. Description of the Prior Art
Recently, considerable interest has developed in a digital-to-analog (D/A) converter that consumes low power for portable voice or audio band products. Generally, a D/A converter converts a digital signal to an analog signal by assigning a voltage weight to each bit in the digital word and summing up the voltage weights of the entire word. To directly implement this conversion process, the D/A converter requires a network of relatively precise components, usually resistors or capacitors.
To avoid the requirement of precise components, the current practice in the design of a D/A converter commonly incorporates a two-level sigma-delta modulator, pulse-width modulator or other similar control devices. The sigma-delta modulator employs the technique of oversampling and one-bit quantization. The sigma-delta modulator generally has a quantizer and a feedback loop. The sigma-delta modulator in a D/A converter receives a digital signal and the quantizer generates output for both the feedback loop and switching circuitry of a load. The feedback control, by taking the difference between this output and input provides an error signal. By repeating this process, the feedback control forces this error toward zero. The output switching circuitry generates an analog signal. The sigma-delta modulator requires fewer number of precision circuits than a conventional D/A converter, and is used for a simpler and more efficient D/A converter design.
The pulse-width modulator converts a digital input code to generate a train of pulse of fixed frequency, with width proportional to the input count. The pulse-width modulator typically comprises a counter, comparator and high-frequency clock. In a D/A converter the pulse-width modulator generates an analog output voltage proportional to the digital input code. This type of D/A conversion is more commonly used when the load is slowly responding system such as a switching regulator, mechanical taped speed servo or electromagnetic controller.
Two-level control devices as mentioned above alternate between the two outputs, High and Low levels, regardless of how small the digital input is. For instance, if the accumulated error is even slightly positive, the control signals for the High level will result. If the accumulated error is even slightly negative, then the control signals for the Low level will result.
One example of a two-level D/A converter is disclosed in U.S. Pat. No. 5,160,896 to McCorkle. The switching circuitry disclosed therein is a class D amplifier including a pair of output MOSFET switches. In a class D amplifier, an active device is used as an on-off switch and output variation is controlled by a modulator or supply voltage source. The output of a class D amplifier circuit, therefore, is derived by switches and there is no gradual translation between different levels. The output voltage alternates between positive and negative levels. As a result, the two-level D/A converter constantly charges its load by either positive or negative voltages and requires an uninterrupted power supply. Such requirements cause constant power consumption and thus inefficient use of power. This can be a serious burden especially for portable voice or audioband products, the appeal of which often depends on a cost effective and compact power supply or a long-lasting battery.
Three-level D/A converters have also been proposed. For instance, U.S. Pat. No. 5, 206,648 to Yukuwa discloses a three-level D/A converter which comprises switches and capacitors between these switches. In such a configuration, the load always draws power from the voltage source regardless of the level of the control signals. Similarly, a three-level D/A converter disclosed in U.S. Pat. No. 5,274,375 to Thompson also includes switching capacitors which constantly draw current from the source. Moreover, the Thompson switching capacitor requires two separate voltage sources providing the same voltages with opposite polarities. This is a difficult requirement to meet and often causes distortion in output unless a significant effort is given for calibration of a voltage supply.
The apparatus according to the present invention provides an improved D/A converter which avoids the above noted disadvantages.